Officers from Environment Agency Wales are on site at an incident in Llwynypia whereby a collapsed wall has damaged a foul sewer, causing sewage to discharge into the Rhondda Fawr.
Dwr Cymru Welsh Water has been called to the site to undertake emergency repairs to stop the sewer from discharging into the river.
Agency officers are monitoring conditions in the Rhondda Fawr closely.
A spokesperson from Environment Agency Wales said:
“Approximately 1.5km stretch of the river is currently affected by the discharge and is appearing discoloured with sewage.
“We’re keeping a close eye on the water quality but so far we haven’t seen any signs of fish struggling.
“We’ll continue to have a presence at the site and will monitor the river until we are sure that the repairs have been made and the discharge has stopped.”
Agency officers last week began emergency work to reduce flood risk to around 200 properties caused by a recent landslip in the area.
The collapse of the wall will not pose a further flood risk to nearby properties. The wall is not an asset of Environment Agency Wales or Dwr Cymru Welsh Water.